In recent years, thin-film transistors (TFT) using an oxide semiconductor film of InGaZnO as an oxide of indium (In), gallium (Ga), and zinc (Zn) or the like have been developed. The InGaZnO thin-film transistor can be fabricated easily even at low temperature and the mobility thereof is known to be high at 10 cm2/Vs or more. Thus, the realization of BEOL (back end of line) transistors embedded in multilayer interconnection is expected. In addition, the application thereof to the next-generation liquid crystal panels is expected.
In contrast to silicon (Si)-MOSFET (metal oxide semiconductor field-effect transistor) and Si-TFT, InGaZnO-TFT is known to have a channel region as an accumulation layer, instead of an inversion layer. If the channel length becomes shorter with increasingly finer patterns, a current path is formed not only in the channel region near an interface on a gate dielectric film side of an InGaZnO film, but also near the interface with a dielectric film on a rear surface side of the InGaZnO film away from the gate dielectric film, posing a problem that a rear surface current flows. Such a problem is caused particularly when the InGaZnO-TFT is operated near a threshold voltage where the gate electric field is small. It is difficult to control a current flowing on the rear surface of the InGaZnO film by the gate electrode. As a result, problems such as degradation of sub-threshold swing S (Sfactor), an increase of off current, and degradation of 1/f noise characteristics are caused.